Method for producing prepolymers containing isocyanate end groups, incorporating a small proportion of base isocyanates

ABSTRACT

This invention relates to a process for producing prepolymers, which contain isocyanate terminal groups and which have a very low content of free isocyanate starting materials, from polyester polyols and polyisocyanates.

This invention relates to a process for producing prepolymers, whichcontain isocyanate terminal groups and which have a very low content offree isocyanate starting materials, from polyester polyols andpolyisocyanates.

Prepolymers which contain isocyanate terminal groups and which areformed from polyisocyanates and polyesters which contain hydroxy groupsare produced and used industrially on a large scale for very differentpurposes, e.g. for the production of cast PU elastomers.

For environmental reasons it is desirable to produce prepolymers whichcontain isocyanate groups and which only have a low content of unreactedpolyisocyanates. This is all the more desirable since these prepolymersare often processed further at elevated temperatures to form finalproducts, whereupon unreacted isocyanate starting materials canvolatilise. Due to the toxicity of the polyisocyanates used, particularsafety precautions are then necessary during the processing of theprepolymers, which increases the cost of processing prepolymers such asthese to form the final products thereof.

According to the prior art, prepolymers such as these are sometimesproduced by separating the prepolymers from unreacted isocyanate bydistillation under vacuum after the reaction of the monomeric isocyanatestarting material with the polyol. When polyester prepolymers areemployed, however, significant amounts of isocyanate still remain in theprepolymer, even after distillation.

The object of the present invention is therefore to produce prepolymerswhich contain isocyanate terminal groups and which have a reducedcontent of monomeric isocyanate starting material compared with theprior art.

Surprisingly, it has now been found that prepolymers such as these canbe produced by very substantially freeing the polyester polyols, beforethey are used, from free polyester components of low molecular weightwhich contain hydroxyl groups, such as diols and triols, and thusachieving a defined content of low molecular weight components in thepolyols to be used.

The present invention therefore relates to a process for producingprepolymers which contain isocyanate terminal groups, which ischaracterised in that polyisocyanates in excess are reacted with apolyester which contains at least two hydroxy groups, of molecularweight 400 to 5000, wherein the polyester used has a content of polyolson which the polyesters are based of ≦6 mol %.preferably ≦3 mol % andmost preferably ≦1.5 mol % with respect to the molar distribution of thepolyester, and the reaction mixture is subsequently subjected todistillation, optionally with the addition of an entraining agent.

The polyesters which are preferably used in the process according to theinvention have a theoretical content of 2 to 4 hydroxyl groups and amolecular weight of 400 to 5000, particularly 600 to 2500, and have acontent of polyols (glycol components) on which the polyesters are basedof ≦6 mol %, preferably ≦3 mol % most preferably ≦1.5 mol %.

According to the invention, the polyisocyanates are used in anequivalent ratio to the polyesters of at least 1.9:1, preferably from2:1 to 20:1, most preferably from 2.2:1 to 10:1.

The content according to the invention of polyols on which thepolyesters are based can be achieved according to the prior art byvarious routes, for example by flash evaporation or thin filmdistillation, whereupon after distillation the polyesters are quenchedvery rapidly, by intensive cooling, to a temperature less than 140° C.,preferably less than 100° C., most preferably less than 70° C., so as toprevent the establishment of an equilibrium comprising the presence ofmonomeric components. Corresponding methods are described in EP 0579988.

All known aliphatic, cycloaliphatic, araliphatic, aromatic and/orheterocyclic polyisocyanates can be used as polyisocyanates in theprocess according to the invention, such as those described, forexample, by W. Siefken in Justus Liebigs Annalen der Chemie, 562, pages75 to 136, with the proviso that it must be possible to distil thesepolyisocyanates under vacuum without decomposition. In general,polyisocyanates which are readily available industrially are mostpreferably used, such as toluene 2,4- and/or 2,6-diisocyanate (TDI).

The aforementioned polyesters which contain at least two hydroxy groupsand which have molecular weights ranging from 400 to 5000 can be used aspolyesters in the process according to the invention. Polyesters such asthese, which contain hydroxy groups, are known for the production ofhomogeneous and cellular polyurethanes, and are described in DE-A 28 32253 for example.

Polyesters which are particularly suitable are those which can besynthesised from commercially available diols and/or triols, whichpreferably contain 2 to 6 carbon atoms, as the polyol component, such asethylene glycol, diethylene glycol, propanediol, butanediol orhexanediol, wherein the carbon chain can also contain oxygen bridgessuch as that in diethylene glycol, and from customary dicarboxylicacids, preferably adipic acid, or industrial mixtures thereof withsuccinic and/or glutaric acid, as the polycarboxylic acid component.

The polyesters which are used can be modified according to the priorart, or can incorporate additives such as plasticisers, emulsifiers orcatalysts.

After the reaction of the polyisocyanates which are used with thepolyesters which are used according to the invention, the reactionmixture is distilled, optionally with the addition of an entrainingagent.

Distillation is usually conducted at temperatures within the range fromabout 130° to 220°, at pressures which range from about 0.001 bar to 1bar.

Entraining agents which are known for purposes such as this, for exampletoluene, can be used as entraining agents.

The most favourable amount of entraining agent to use can easily bedetermined by appropriate preliminary tests, and usually ranges from 2to 5% by weight with respect to the amount of prepolymers.

Prepolymers which contain isocyanate terminal groups, which are producedby the process according to the invention and which have a very lowcontent of isocyanate starting materials, can be further processed inthe usual manner according to the prior art for the production ofpolyurethane plastics (using corresponding polyol synthesis components).

EXAMPLE 1.1 Production and Modification of Polyesters

The polyesters used were in all cases those which were synthesised fromethylene glycol and adipic acid only. They were produced in the mannerknown in the art by heating the two starting materials to about 200° C.under a vacuum of up to 15 mbar, with the water of reaction being takenoff simultaneously. The end point of the esterification was reached whenthe acid number of the reaction mixture had fallen below 1.

In order to remove components of low molecular weight afteresterification, the polyesters were freed from volatile components oflow molecular weight in a flash evaporator at temperatures of 200° C.and pressures of <0.2 mbar. This short-term treatment substantiallyprevented the formation of short-chain species by transesterificationreactions in the polyester. Therefore, the polyesters were no longer ina statistical equilibrium.

Two polyesters with different molecular weights were produced. Theanalytical data obtained for these esters before and after flashdistillation are given in the following Table:

Before distillation: After distillation: Ex. AN OHN Mn Fr. gly AN OHN MnFr. gly 1a 0.6 64.2 1750 0.4 0.5 56.7 1980 <0.02 2a 0.9 54.5 2060 0.20.98 49.0 2290 <0.02 AN = acid number (mg KOH/g), OHN = hydroxyl number(mg KOH/g), Mn = number average molecular weight, Fr. gly = free glycolin % by weight.

Comparative Example A Commercially Available Polyester withoutDistillation of the Short-chain Constituents

Desmophen ® 2000 MZ: AN: 0.9 OHN: 56 Mn: 2000 Fr. gly: 0.2

Desmophene® 2000 MZ is a polyester formed from adipic acid and ethyleneglycol (a commercial product of Bayer AG)

1.2 Production of Polyester-TDI Prepolymers with a Low Monomer Content

In order to produce polyester-TDI prepolymers with a low monomercontent, the polyesters obtained as in 1.1 were reacted in a molar ratioof 1 to 8 with TDI 80 (=a mixture of isomers comprising 80% 2.4- and 20%2.6-TDI) and were subsequently freed from excess TDI in a flashevaporator.

The prepolymers were characterised by the following analytical data:

NCO NCO Viscosity (theor.) free TDI (found) (70° C.) Ex. PE* (%) (% byGC) (%) (mPas) 1b 1a 3.6 <0.03 3.5 3150 2b 2a 3.2 <0.03 3.2 4540 Datagiven as percentages by weight *PE = polyester from Example 1.1 afterdistillation

Comparison

For comparison, a prepolymer was produced analogously to 1.2 from thecommercially available polyester Desmophen® 2000 MZ manufactured byBayer AG. The following analytical data were obtained for thisprepolymer:

NCO NCO Viscosity (theor.) free TDI (found) (mPas) (%) (% determined byGC) (%) (mPas) 3.65 16 3.4 3380 Percentages given as percentages byweight

1.3. Production of Elastomers from the Prepolymers Described in 1.2

The prepolymers described in 1.2 were reacted in the usual manner at100° C. with a crosslinking agent comprising the isobutyl ester of4-chloro-3,5-diaminobenzoic acid to form an elastomer and were cast intomoulds (20×20×2(6, 12.5) mm). The castings were subsequently annealedfor 24 hours at 110° C., and after 14 days the properties listed in thefollowing Table were measured:

TABLE Measured according to Prepolymer 1b 2b Comparison DINStoichiometric ratio of 1.1:1 1.1:1 1.1:1 prepolymer to crosslinkingagent Shore hardness A/D 92/32 90/30 91/32 53505 Tensile strength (Mpa)50 49 40 53504 Impact resilience (%) 41 45 40 53512 Tear propagation 4945 45 53515 resistance (kN/m) Abrasion loss (mm³) 46 43 40 53516

Results

It can be seen from the Table of prepolymers that the commerciallyavailable products have a considerably higher content of free TDI thando the products according to the invention, in which the free TDI wasless than the limits of detection of the methods used.

It can be seen from the Table of elastomers that the mechanicalproperties of the elastomers were not affected by the pretreatmentaccording to the invention of the polyesters.

What is claimed is:
 1. A process for the production of a prepolymercontaining terminal isocyanate groups comprising a) reacting 1) anexcess of polyisocyanate with 2) a polyester having at least two hydroxygroups and a molecular weight of from 400 to 5,000 in which the polyolcontent is no greater than 6 mol % of the total moles of polyester andb) distilling the reaction product of a), optionally with the additionof an entraining agent.
 2. The process of claim 1 in which the polyesterhas a polyol content no greater than 3 mol %.
 3. The process of claim 1in which the polyester has a polyol content no greater than 1.5 mol %.4. The process of claim 1 in which the polyisocyanate and polyester arereacted in amounts such that the equivalent ratio of polyisocyanate topolyester is at least 1.9:1.
 5. A process for the production of a castpolyurethane elastomer comprising a) reacting an excess ofpolyisocyanate with a polyester having at least two hydroxy groups and amolecular weight of from 400 to 5,000 in which the polyol content is nogreater than 6 mol % of the total moles of polyester, b) distilling thereaction product of a), optionally with the addition of an entrainingagent, and c) reacting the product of b) with a crosslinking agent.